This piece, authored by Dr Brett Glencross, was first published in International Aquafeed - August 2025 edition.

Whilst navigating the oceans to secure our next seafood meal, fishing vessels employ a wide range of various fishing techniques. Among those techniques employed, trawling is one of the more widespread, but not necessarily the main source of all seafood. Most of us will have by now seen the “Oceans” documentary presented by David Attenborough where they vilify this fishing technique as one of ecosystem destruction and overfishing. However, reality is not as black-and-white as the documentary makes out and it is important to distinguish between benthic-trawling, mid-water trawling and other fishing methods in terms of their impacts. Not all fishing has the same impact on the environment. It is also important to note that bottom trawling makes up ~25% of total global fish catch. Mid water trawl accounting for another ~10%. Whereas purse seine fishing, mostly used by the marine ingredients industry, accounts for another ~20%. So, combined these three net-based methods account for more than half of all fish caught in one way or another.

While both benthic and mid-water trawling aim to harvest marine life through towing a net through the water, their distinct methodologies lead to substantially different ecological footprints. Both trawling methods have a high use of fuel to catch landed. Purse seining by contrast works by a net being encircled around a shoal of fish and it then being closed around them and hauled in. It has among the lowest use of fuel to catch landed. Generally, the by-catch also diminishes with purse seine < midwater trawl< benthic trawling. Similarly, the interaction with the broader ecosystem also has the same hierarchy. Understanding these differences is crucial for assessing the sustainability implications of each technique.

Benthic trawling, as its name suggests, involves dragging heavy, weighted nets along the (benthos) seafloor. The method requires high intensity fuel use (>1000L fuel per tonne of catch) to pull the net through the water, resulting a comparatively high carbon footprint. This method is highly effective for catching benthic species—those that live on or near the bottom, such as prawns, haddock, and flatfish. However, the heavy gear has clear interactions with seabed habitats. Some of those habitats, like coral reefs, sponge gardens, and seagrass meadows, are relatively sensitive and slow to recover environments, and can be damaged in a single benthic trawl pass. Notably, many benthic trawl sectors deliberately avoid these habitats as they are also damaging to gear as well. Furthermore, bottom trawling is notorious for its high rates of bycatch—the unintentional capture of non-target species. Many of which are discarded, either dead or dying.

In contrast, mid-water trawling (also known as pelagic trawling) targets schooling fish that swim in the open water column, such as herring, mackerel, and tuna. These nets are designed to remain suspended above the seafloor, minimizing any contact with benthic habitats. This distinction is important, as it avoids the potential physical destruction associated with bottom trawling. Many mid-water trawls also happen in oceanic situations requiring fishers to motor hundreds of kilometres out to sea to get to the fishing grounds. Notably, mid-water trawling is not without its own set of environmental concerns. The fuel use intensity is less than that of benthic trawling, but still among the higher range of fishing methods. The notion of towing a bag through the water behind a boat creates a lot of drag and uses a lot of fuel. While not directly damaging the seabed, they can lead to significant localized depletion of target fish stocks. Bycatch remains a problem, albeit generally lower than in bottom trawling, and can still impact vulnerable species like marine mammals and seabirds that encounter the vast nets.

Different again is purse seining. Purse seining is the method used to target some of the world’s largest single-species fisheries (e.g. Peruvian anchoveta). It typically has no impact on the seafloor as it targets only those species near the sea surface. Because it uses low fuel intensity and has relatively large yields, and happens mostly within coastal shelf regions, meaning fishers don’t travel too far to access the fish, it has a very low fuel to catch landed (<100L fuel per tonne of catch). It can however lead to significant localized depletion of target fish stocks and has also been linked to effects on the marine food web, affecting predators that rely on those schooling species for sustenance. Bycatch is usually less of a problem, than in trawling, but it can still impact some species like marine mammals and seabirds that encounter the vast nets. Notably, most mammal and bird species caught in purse seine nets can be released unharmed.

Ultimately, all fishing methods have impacts, but the nature of the impacts differs significantly. Benthic trawling's direct physical interaction with habitats and high bycatch rates poses a potential threat to biodiversity and ecosystem function. Mid-water trawling, while less destructive to the seabed, still has some level of bycatch, impacting the balance of open-ocean ecosystems. Purse seining has less impact again. Importantly though the technique used depends much on the species targeted. Benthic trawling wouldn’t work on Peruvian anchoveta, and purse seining wouldn’t on work prawns. So, for some species we are limited to certain techniques to catch the target species. For the marine ingredients industry most of the fish is caught using purse seine and mid-water trawl techniques, though by-product sourced raw material comes from fish caught for human consumption using all the various fishing techniques used.

Regardless of the fishing technique, overfishing remains a threat to the oceans, which is directly linked to governance and regulation. Moving forward, a holistic approach to fisheries management is essential. There needs to be a focus on clearer regulations and better communication to all stakeholders. There is still room for improved gear selectivity and protection for vulnerable habitats. And most importantly there is a need for greater engagement with the fishing community. Ultimately, they are the biggest stakeholder and the ones whose livelihood depends on getting this right, and with the potential to make the biggest impact for the better.